The use of wind turbines has become a common way of generating electricity and the size and performance of the turbines have increased.
The wind turbines typically include multiple blades which transform wind energy into rotational motion of a drive train to thereby produce electricity. Existing turbines are controlled by computerised controllers which can amend various settings to bring the turbine to an optimum with regards to power production, load on the blades and drive train and general wear on the turbine.
Often, the blades can rotate about their longitudinal axis and thereby convert a varying degree of the wind energy. This activity is referred to as “pitching”, and in a traditional wind turbine, pitching is controlled such that the wind turbine utilise as much of the available wind energy until a rated power production is reached. If the wind turbine has reached the rated power production and the available wind energy increases further, the blades are pitched away from the optimal situation to maintain the rated power production. At a certain cut-out wind energy intensity, the blades are pitched to a position where transformation between wind energy and rotational motion is prevented. This is often referred to as “feathering”. The rotation stops and the control system waits for a decrease in wind intensity before reinitiating production by pitching back the blades from the feathered position.
The loads on the wind turbine structure are highly dependent on the climate conditions in which the turbine operates and the size of the major components e.g. blades. Different control algorithms are deployed today on the wind turbines to reduce the loads based on climate conditions.
Asymmetrical loadings across the rotor are responsible for a significant contribution to fatigue loads. Asymmetrical loadings are caused e.g. by wind shear, and today wind turbine controllers are sometimes adapted to reduce or eliminate tilt and yaw moments on the wind turbine by controlling the pitch of each blade separately. This activity could be based on conditions, e.g. blade bending, experienced individually by each blade. Sometimes, this is referred to as Yaw and Tilt Control. In practise, the asymmetric loads are balanced by cyclic pitching of the blades based on estimated/calculated tilt and yaw moment on the rotor, and in the existing controllers, the loads are brought to a predefined, static, reference level also referred to as a threshold value. The static threshold value is typically defined during the design phase based on the structural limitations of the wind turbine, based on the risk of tower-blade collision etc.
US2011/0064573 discloses a method for controlling operation of a wind turbine where a set point limit is determined based on a measured atmospheric condition.
US2006002792 discloses a method for reducing load and providing yaw alignment in a wind turbine includes measuring displacements or moments resulting from asymmetric loads on the wind turbine. These measured displacements or moments are used to determine a pitch for each rotor blade to reduce or counter asymmetric rotor loading and a favourable yaw orientation to reduce pitch activity. Yaw alignment of the wind turbine is adjusted in accordance with the favourable yaw orientation and the pitch of each rotor blade is adjusted in accordance with the determined pitch to reduce or counter asymmetric rotor loading.
In an attempt to reduce asymmetric loading, the blades are sometimes pitched unfavourably with regards to conversion of wind energy to rotational movement, and the traditional Tilt and Yaw control therefore potentially reduces the power production of the turbine. Tilt and Yaw control by pitching also increases the wear on the pitching system and thereby increases the maintenance costs.